The most frequently used sulfonamide is sulfamethazine (SMZ) because it is often found in foods made from livestock, which is hazardous for individuals. Here, we have developed an easy, quick, selective, and sensitive analytical technique to efficiently detect SMZ. Recently, transition metal oxides have attracted many researchers for their excellent performance as a promising sensor for SMZ analysis because of their superior redox activity, electrocatalytic activity, electroactive sites, and electron transfer properties. Further, Cu-based oxides have a resilient electrical conductivity; however, to boost it to an extreme extent, a composite including two-dimensional (2D) graphitic carbon nitride (g-C₃N₄) nanosheets needs to be constructed and ready as a composite (denoted as g-C₃N₄/Cu₂Y₂O₅). Moreover, several techniques, including X-ray diffraction analysis, scanning electron microscopy analysis, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy were employed to analyze the composites. The electrochemical measurements have revealed that the constructed g-C₃N₄/Cu₂Y₂O₅ composites exhibit great electrochemical activity. Nevertheless, the sensor achieved outstanding repeatability and reproducibility alongside a low limit of detection (LOD) of 0.23 µM, a long linear range of 2 to 276 µM, and an electrode sensitivity of 8.86 µA µM⁻¹ cm⁻². Finally, the proposed GCE/g-C₃N₄/Cu₂Y₂O₅ electrode proved highly effective for detection of SMZ in food samples, with acceptable recoveries. The GCE/g-C₃N₄/Cu₂Y₂O₅ electrode has been successfully applied to SMZ detection in food and water samples.